Applications for reinforced thermoset composites are currently limited by manufacturing technology. This is particularly true in industries where the use of light, high performance composites is important and/or critical. There is considerable interest in refining existing technologies and/or developing new composite processing techniques.
Composite processes may generally be considered as using either open or closed molds. Only closed mold techniques are discussed here because they have advantages in surface finish, capability for detailed configurations, and part to part uniformity. However, it will be realized that the discussion applies equally to open mold techniques.
Most thermoset composites are based on either polyurethanes, phenolics, polyesters, or epoxies. Of these, polyurethanes and polyesters are the most widely used due to their lower cost. Fillers are added to improve mechanical properties and aid processing. Common fillers include glass fibers, which improve mechanical properties, and mineral fillers such as calcium carbonate which reduce cost and improve flow characteristics during molding, and the like. Other fillers are frequently added but fibers and mineral fillers are used most widely and in the greatest concentrations. Based on final properties and/or cost, it may be desirable to incorporate up to 60-70 percent by volume of these components.
Processing techniques aim to produce accurate, high quality parts from these compositions in minimal cycle times. It is also desirable to maximize the filler loadings a process can handle which will increase the variety of parts that can be made.
One of the fastest growing closed mold processes is reaction injection molding (RIM). RIM is a process where reacting components are mixed intimately and loaded into molds. Most of the resin systems currently used are based on polyurethane. Extremely fast cure times, on the order of several seconds, are possible due to the high reaction rates involved. Reinforced RIM, i.e. RRIM, refers to modifying the conventional RIM process by adding reinforcements, usually glass fibers. Unfortunately, the development of this process appears constrained by inherent couplings between the mixing, forming, and curing stages.
The RRIM is limited by the amount of reinforcements that can be added. The usual procedure is to mix the fillers into the unreacted components prior to impingement mixing. The ability to pump these very viscous slurries and still obtain a high Reynolds number at the mix head restricts fiber loadings. Incorporating the fibers at any point after impingement is constrained by the fact that rapid reaction times require correspondingly rapid mixing and fiber wetting. In this respect, processes which remove the coupling between mixing and curing have a clear advantage over RRIM.
RRIM is also constrained in that there are no techniques, other than gate positioning, which exist for controlling the orientation of fibers. In this case the rapid reaction rates couple the curing and forming stages, restricting the possible forming operations. Besides limiting applications involving chopped glass fibers, this coupling particularly hinders the development of RRIM in the area of continuous filaments.
Another approach to composite processing is that used with sheet molding compound (SMC). In this process the resin, fillers (except glass fibers), and processing additives are mixed together in batches. A layer of the resulting paste is applied continuously to a polyethylene film. On top of this paste layer glass fibers, either chopped or continuous, are added. A second paste layer is applied on top of the fibers and the sandwich pressed together to form the SMC molding material. The resin, usually polyester, uses a heat activated chemistry. This decouples the mixing and curing stages, removing the problem of incorporting large amounts of reinforcements that limits RRIM. The SMC molding charge is placed in hot steel molds which simultaneously form the part and heat the resin, initiating the cross-linking reaction. The coupling between forming and curing in this process may be partially responsible for problems involving surface finish and the inability to make sections less than 40-50 thousandths of an inch thick. In addition, the conduction heating method used causes undesirably long cycle times.
A number of causes for poor SMC surface finishes have been proposed. These include nonuniform cure induced flow distortions while forming, residual stress, shrinkage in resin enriched areas, and nonuniform filler concentrations in the molding charge. The last two causes may be resolved by using low profile additives and closely controlling the composition of the molding charge. Cure induced flow distortions may be eliminated by forming in a cold mold and then heating. If the heating is applied uniformly, this approach may also permit uniform, residual stress-free curing. In constrast, conventional conductive heating inherently causes nonuniform curing and leaves residual stresses.
SMC's inability to make thin sections may be caused by either flow separation or premature curing during forming. In flow separation, only the resin flows leaving the fillers behind. One explanation contends that flow separation occurs when the resin viscosity is too low to carry the fillers in high shear conditions. Noting that temperature increases cause the resin viscosity to decrease, flow separation will be most prominent in thin sections where both shear and heat conduction from hot molds is worst. Similarly, premature curing which can prevent some section from filling is caused by excessive heating during forming and is worst in thin sections. The use of cold molds to decouple the forming and curing operations will eliminate unwanted heating and may allow thinner sections to be made.
Compared to RRIM, SMC has relatively long cycle times on the order of 11/2-10 minutes. A number of costs are associated with these long cycle times including capital costs (a typical SMC press may cost one million dollars), tooling costs, and the labor costs connected with running a press. Shortening cycle times will reduce all of these costs on a per part basis. Unfortunately, SMC cycle times are restricted by the required conductive heating. Conductive heating rates are intrinsically limited by the thermal conductivity of the composite, the requirement that the mold be cooler than the composite's degradation temperature and the requirement that minimal curing take place during forming.
Other closed mold composite processes include transfer molding, bulk molding compound (BMC), thick molding compound (TMC), foam reservoir molding (FRM), and injection molding. These processes are all similar in that they contain innate coupling, similar to those discussed for SMC, which potentially limit development.
Electric field heating, particularly dielectric, has been used for many years in the polymer industry. Dielectric heating has been used to preheat charges prior to transfer molding, as a booster heater in oven cures, to melt and cure adhesive layers, and to cure resins in the pultrusion process. Microwaves have also been used, primarily in drying, but also to a limited degree in curing processes. Some specific applications where microwave drying has been applied include films, powders, filaments, foams, and pellets. In these applications, dielectric and microwave heating have been used to reduce cycle times by replacing or augmenting a limiting aspect of the existing process. For instance, U.S. Pat. No. 3,816,574 describes the use of an alternating magnetic field to heat and produce expanded foam articles. U.S. Pat. Nos. 3,640,913; 3,104,424; and 3,217,691 describe methods for preparing polymer particles for use in producing foamed polymers using high frequency alternating electric fields. U.S. Pat. No. 3,848,038 describes the use of microwave energy to dry expanded plastic.
Other uses of electric field energy in treating polymers include thermosetting of polyurethane or polysulfide sealing compositions containing particles having a dielectric constant greater than 200 as described in U.S. Pat. No. 3,936,412 and bonding plastic materials having magnetic particles at the interface to intermix materials at the interface and improve bond as described in U.S. Pat. No. 4,035,547.
The present invention involves the use of high frequency electric field to heat and cure polymeric systems, particularly for producing reinforced thermoset composite articles.
Electric field heating, to contrast in conduction heating, has very low thermal inertia permitting close control of the reaction rate. Conduction heating may be removed entirely from the dielectric process by using cold molds. Resulting advantages of heating with only electric fields include reduced cycle times, minimal heat inputs, and decoupling of forming and curing operations. Reduced cycle times result from the ability to put the required heat to initiate reaction uniformly into the resin mixture at a very fast rate, limited only by the electrical breakdown strength of the material. For the reasons noted in the discussion of SMC above, conventional conduction heating rates are inherently limited. Minimal heat inputs are possible with electric fields by heating only the part and by turning the field off once curing has been initiated. With conduction heating, unwanted heat losses continue through the curing stage and after the part has been removed from the mold. The advantages of decoupling the forming and curing steps are possible when forming in cold molds, followed by electric field initiated curing of the formed part. This approach effectively removes the time constraint from forming operations, simplifying the processing of a variety of compositions and geometries. In contrast to the conventional SMC process, uniform residual stress-free cures are possible because the curing occurs from inside. The fact that the outer surface will cure last can also result in improved surface finishes. Furthermore, since the flow of materials occurs independent of the curing process, thinner sections may be made and the thermoset chemistry modified to react at lower temperatures. This results in further gains in minimizing cycle times and reducing heat inputs.
Preferentially heating the composite system near localized internal heat sources also helps to minimize the required heat input and reduce cycle times. By such preferential heating of only part of the system, relatively low final temperatures are possible even in cases where the resin is initially heated to 200.degree. C. This ability to heat only the resin to high temperatures, but stay below the composite's degradation temperature after the chemical exotherm has been generated and temperature nonuniformities distributed, reduces cure times.
Both electric field heating and the concept of propagation from heat sources can improve mechanical properties. As mentioned above, electric field techniques permit uniform heating and curing which will produce parts with negligible residual stress. This is the opposite of conventional conduction processes which cure from the outside in and innately generate residual stress fields which may later cause warping or accelerate failure. A further mechanical advantage is gained from the propagation concept. Propagating the chemical reactions outward from lossy fillers in accord with the present invention causes the resin to shrink around the fillers. For example, a typical polyester resin has a six to seven percent volume shrinkage. Shrinking around fillers improves resin/filler adhesion, enhancing mechanical properties.